1. Field of the Invention
The present invention relates generally to computer systems, and more particularly, to a method and apparatus for reducing electromagnetic interference in a printed circuit board, or the like, of a computer system.
2. Discussion of the Related Art
Discontinuities in signal paths of high speed return currents on a printed circuit board are a potential source for generation of electromagnetic interference (EMI) radiation and noise coupling. In addition, EMI radiation and noise coupling can cause undesirable adverse operation of circuit on the printed circuit board. A method and apparatus for reducing the undesired EMI interference and noise coupling is thus desired.
With respect to multilayer printed circuit boards, for example, as shown in FIG. 1, segmenting of a conductive layer (10, 12) on an insulative layer 14 of the multilayered printed circuit board 16 may be done. Segmenting involves the dividing up of a planar conductive layer into physically separated segments, for example, segments 10 and 12. In other words, each segment is physically separated from one another by a void or an insulative material 18. The segments could be electrically connected via a capacitor or the like. A typical conductive plane which is segmented includes, for example, copper (Cu).
The segmenting of the conductive layer on a plane can be implemented for various reasons. One reason may include providing a power or reference plane having two different voltages. For example, a first segment 10 may be used to carry a 3.3 v reference voltage. A second segment 12 may be used to carry a 5.0 v reference voltage. In such a situation where the two segments are at different voltages, the segments must be physically separated. In other words, a physical void or insulative material 18 exists between segments. Multilayered printed circuit board often refers to printed circuit boards having two or more conductive circuits, wiring or segmented layers separated by one or more insulative layers 20, 22. Segmented layers could be included on any one of the conductive layers, as needed for a particular printed circuit board implementation.
While segmenting has been discussed with respect to a voltage plane, a ground plane could be segmented also. In such an instance, one ground plane segment could represent a ground plane for digital circuitry, the digital circuitry being characterized as noisy. Another ground plane segment could represent a quiet ground plane. The two ground plane segments are physically separated to maintain their respective characteristics, i.e., so that noise from the noisy ground plane segment does not bleed into the quiet ground plane segment. As discussed herein, plane segmentation can be done for a power plane, a ground plane, or any other reference plane.
In addition, to the above, multilayer printed circuit boards include several layers of laminated material, for example, layers 24, 26 including conductive and insulative materials. Any one layer may include one or more reference segments, signal lines, and/or circuit portions. Furthermore, a single layer can include more than one segment. As discussed, the segments of any particular layer may include voltage or power plane segments, ground plane segments, or any combination of reference plane segments. A signal layer can also include a reference segment on the signal layer.
A problem arises when there are two different segments on a given plane of a multilayer printed circuit board 16 and an interconnect 28 in a second plane traverses over a boundary of a first segment 10 and a second segment 12. The interconnect 28 can be situated in a layer above or below the first and second segments. Furthermore, the interconnect 28 is separated from the first and second segments by an insulative layer material 20. If we assume that a driver 30 is situated on the side of the first segment 10 and connected via the interconnect 28 to a receiver 32 situated on the side of the second segment 12, then a signal current, I.sub.s, is driven through the interconnect 28. As the signal current travels down the interconnect 28, there are two things that happen. First, the impedance of the interconnect 28 determines how smoothly the signal current I.sub.s will travel down the interconnect across the underlying segments. Note that the segments may alternatively be overlying segments. Secondly, considering for a moment small crosssections of the interconnect, from the driver 30 to the receiver 32, the impedance of the individual cross-sections drastically changes in the region of the void 18 between the first segment 10 and the second segment 12. In other words, a portion of the interconnect in the region of the void 18 between the first segment 10 and the second segment 12 encounters a drastic change of impedance.
Over the first segment 10, the interconnect impedance is referenced with respect to the first segment. As a high speed or high frequency signal travels from the driver 30 to the receiver 32, two things occur. That is, first, there is a change in impedance in the region of the void 18 between the first segment and the second segment. Such a change in impedance will have an adverse effect upon the high speed signal current, I.sub.s, and the corresponding voltage waveform that traverses the interconnect. The high speed signal may include, for example, a 6 MHz, 8 MHz, 33 MHz, 66 MHz, 100 MHz, or any other, repetitive, periodic, or pseudo-periodic signal having a high frequency. A pseudo-periodic signal is characterized by a signal that appears periodic for certain durations and non-periodic for other durations. Secondly, in response to the signal current, I.sub.s, there exists a return current, I.sub.r which travels along the segments of the reference plane. That is, when the signal current, I.sub.s, travels down the interconnect 28, there is a return current, I.sub.r.
The return current I.sub.r is dissipated along the return path through the segments into various return currents and loop currents as shown in FIG. 2. Considering a cross-section of the interconnect 28 above the second segment 12, the return current (density) follows a normal distribution curve just under the cross-section of the interconnect. The majority of the return current for the high speed signal, will reside underneath the interconnect above the second segment. The return current will try to follow the route of the interconnect 28, which is true for high speed signals but not true for DC signals. In other words, the return current, I.sub.r, tries to return to the driver 30 or source via the segments of the reference plane. As the return current, I.sub.r, reaches the void 18 between the second segment 12 and the first segment 10, the return current, I.sub.r, encounters a "brick wall." The "brick wall" represents the void 18 where there is no physical connection between the second segment 12 and the first segment 10. The majority, or a high magnitude, of the return current will try to go across a face of the respective segment. In essence, however, the majority of return current creates a loop current i.sub.L1 to each side of the interconnect 28 within the second segment 12. A pitfall of such a created loop current is that any circuit elements or circuits in the proximity of the created loop current, above or below, can be adversely affected in an undesirable manner. The loop current i.sub.L1 is created because the return current I.sub.r cannot couple from the second segment 12 to the first segment 10 and go back to the source 30 (i.e., the driver). Undesired coupling of the loop current with circuit elements or circuits in a proximity of the loop current in one or more adjacent layers can thus occur. With electromagnetic interference (EMI), if a cable attachment (not shown) is in proximity to the loop current i.sub.L1 the cable extending perhaps out to or from a chassis, connector, keyboard, or other device, then the loop current i.sub.L1 could undesirably couple onto the cable. Coupling of the loop current onto the cable can result in the cable acting as an antenna, the loop current acting to drive the antenna. Unwanted EMI noise is thus added to the operation of the nearby circuit or signal line, whatever the circuit or signal line happens to be. Given that the signal I.sub.s of the interconnect 28 is a high speed periodic or pseudo-periodic signal, EMI noise created as a result of the strong loop current i.sub.L1 can be detected outside the multilayer printed circuit board 16. In other words, a cable or wire being driven by the fairly strong loop current acts as an antenna and starts radiating EMI noise. In a given frequency range, the EMI noise can be detected with a receiver, such as at the fundamental or a harmonic of the high speed signal frequency.
A very small portion of the return current will couple I.sub.r ' onto the first segment 10. The relative strengths of the return loop current i.sub.L1 and coupled return current I.sub.r ' are illustrated with a solid line and dotted line, respectively, as shown in FIG. 2. The magnitude of return current I.sub.r ' which is coupled onto the first segment is much less or at a lower magnitude than the return loop current I.sub.r. The coupling of current from the second segment to the first segment is due to an inductive coupling that exists between two parallel planes, that is, a mutual inductance. Secondly, a coupling of the return current is also due to a mutual capacitance that exists between the second segment and the first segment. The current I.sub.r ' which is coupled to the first segment 10 will travel to the source or driver 30 to complete the return loop, i.e. return current signal from the receiver to the driver.
As mentioned, a main problem with the embodiment as shown in FIG. 2, is that any cables and/or circuits proximate to the region of the void between the second and first segments can be adversely affected or undesirably influenced. That is, any circuits and/or cables proximate the return current loop i.sub.L1 (of return current that has not coupled onto the first segment) will be adversely affected. Still further, spurious undesired noise can adversely affect circuits in the region proximate the void and elsewhere on the multilayer printed circuit board. The problem may include either a functional problem or an EMI problem, or both.
While the first and second segments could be capacitively coupled to one another via a discrete capacitor, such capacitive coupling may not always be feasible and/or desired. Capacitive coupling furthermore adds to the expense of manufacturing of a particular multilayer printed circuit board. A solution for EMI reduction without the use of discrete capacitors is desired.